An acrylic sheet such as a pressure sensitive adhesive sheet is commonly produced by a process comprising dispersing an acrylic polymer component in a solvent or dispersing an acrylic polymer component in a dispersion medium, coating a substrate with the solution or dispersion and removing the solvent or dispersion medium from the coating layer.
The solvent or dispersion medium used in the above process is removed after the coating. Therefore, not only is a relatively high cost required for the use, removal and recovery of the solvent or dispersion medium but also there is the danger of working environment pollution depending on the type of solvent or dispersion medium.
In these circumstances, there have been proposed processes in which, without the use of any solvent or dispersion medium, a monomer is applied onto a substrate and polymerized on the substrate.
For example, Japanese Patent Laid-open Publication No. 50(1975)-102635 discloses a pressure sensitive adhesive tape in which a mixture of an acrylic monomer and a thermal polymerization initiator is interposed in the form of a thin layer between a tape support and a release sheet and heated and polymerized in a closed system so that a semisolid pressure sensitive adhesive layer is formed therefrom.
When an acrylic monomer of low viscosity is applied in the form of a thin layer and thermally polymerized employing thus the technique disclosed in the above publication, because of the thinness of the layer, not only is the calorific value of polymerization small but also the heat of polymerization is effectively dissipated due to heat radiating from the surface of the tape support. As a result, the tape temperature is rarely higher than the temperature achieved when the reaction is initiated. That is, the monomer reaction temperature on the support is usually lower than the heating temperature for inducing the reaction. In the above application of the monomer onto the support in the form of a thin layer, the value of dissipated heat is generally greater than the calorific value of the polymerization reaction, so that the tape shrinkage and deformation by exotherm seldom occurs. Moreover, the reaction can be performed while controlling so as to avoid the support deformation, etc., thereby enabling the effective reduction in the amount of residual monomer.
On the other hand, apart from the above thin sheet, the sheet with a thick adhesive layer is advantageous, for example, because the stress at bonding can be absorbed to thereby enable maintaining a large bonding strength for a prolonged period of time and hence ensuring an enhanced bonding reliability in addition, when bonded to an adherend with an uneven surface, the thick adhesive layer sheet exhibits a high capability of following the uneven surface.
It is difficult to form this thick adhesive layer by the application of a monomer having a low viscosity. Even if a monomer is thickly applied, the calorific value generated by polymerization reaction is greater than the value of the heat spontaneously dissipated from the support because of the large thickness of the layer to thereby render the control of reaction temperature difficult.
The above reactive adhesive layer or the like is often applied onto a film of a synthetic resin such as a polyester resin as a support. Thus, when the reaction temperature is extremely high, the support is likely to suffer from thermal distortion or thermal shrinkage.
Therefore, in the formation of a layer whose thickness is greater than a given value, use is made of, for example, method (1) in which an acrylic adhesive dissolved or dispersed in a solvent is divided and applied a plurality of times, method (2) in which ultraviolet rays permitting a low temperature rise at reaction are irradiated to thereby effect polymerization, method (3) in which a screw extruder is employed and method (4) in which, for restricting the temperature rise during reaction, polymerization is performed while passing the reaction mixture through a heat buffer whose heat transfer coefficient is large (see Published Japanese Translation of PCT Patent Applications from Other States, No. 9(1997)-512054).
However, in the method (1), the coating thickness obtained by one coating operation is limited, so that it is required to pile thin-layer sheets one upon another, this resulting in a cost increase. Further, because of the use of a solvent, problems are encountered such that, when the molecular weight is large, the coating liquid has a high viscosity to thereby render the coating operation difficult and such that the use of a crosslinking agent is restricted. The method (2) is advantageous in that the use of ultraviolet rays enables the polymerization at room temperature to thereby enable controlling the heat of polymerization generated in the internal part of the sheet layers. However, the transmission of ultraviolet rays through the internal part of the sheet becomes difficult in accordance with the increase of the sheet thickness. Accordingly, the sheet surface at which the incident ray intensity is the maximum and the internal part or bottom part thereof have different polymerization rates with the result that it is difficult to produce a sheet of uniform thickness. In particular, when the sheet contains a filler through which ultraviolet rays cannot be transmitted, it is practically infeasible to employ this method. Further, for completing the polymerization, the period of ultraviolet irradiation must be prolonged to thereby increase both facility cost and production cost. Still further, although the technique of using an ultraviolet polymerization initiator and a thermal polymerization initiator in combination in order to treat residual monomers has been disclosed (see, for example, Japanese Patent Laid-open Publication No. 5(1993)-5014), this technique also invites facility cost and production cost increases. The method (3) comprises controlling the molecular weight of partial polymerizate by means of a screw extruder to thereby obtain a polymerizate whose conversion of monomers into a syrupy composition is about 90% and coating a sheet with the obtained polymerizate to thereby obtain a coated sheet. In this method, the fluidity must be ensured for conducting the polymerization reaction at high temperatures in a screw extruder. Thus, because the formation of a crosslinking structure in a polymerizate leads to an extreme viscosity increase, any crosslinking structure cannot be formed in the polymerizate in the screw extruder. Consequently, steps such as irradiating a material obtained by bulk polymerization with electron beams or sheeting a mixture of a polyfunctional monomer and a sheet forming material followed by irradiating the sheet with electron beams, radiation, ultraviolet rays or the like to thereby introduce a crosslinking structure are inevitable. The method (4) is advantageous in that the polymerization is performed in a heat buffer whose heat transfer coefficient is large while controlling the temperature of reaction mixture substantially within 20.degree. C. to thereby enable imparting desirable properties to the adhesive sheet. However, there are problems such that it is difficult to complete the polymerization within a short period of time to thereby disenable the use of currently available heating oven drying equipment.
As apparent from the above, not only is the production of a thick sheet or thick adhesive sheet per se conspicuously difficult, but also a thick acrylic sheet or thick acrylic adhesive sheet having high uniformity and free of the distortion of support, etc. cannot be produced at a low cost using conventional processes.
The present invention provides a process for easily producing a thick sheet or thick adhesive sheet at low cost.
The present invention further provides a thick acrylic sheet or thick acrylic adhesive sheet having high uniformity and free of the distortion of support, and the like.